It is known in the art relating to fixed geometry mixed and radial flow dynamic gas compressors, generally referred to as centrifugal compressors, that the differential pressure, or pressure ratio, across a compressor, the efficiency and the operating flow range as a percentage of the maximum or choke flow are determined in part by the type and geometry of the diffuser used in the assembly. In general, so called vaneless diffusers provide the highest operating range but the lowest maximum pressure ratio and efficiency. Diffusers with special air foil shaped vanes improve the maximum pressure ratio and efficiency with some reduction in the operating range. Finally, diffusers with generally wedge shaped straight sided blades, referred to as the straight island type, generally provide the highest pressure ratio and efficiency at the expense of still further reduction in the operating range.
Mechanically variable geometry diffusers for centrifugal compressors have been considered in the past to provide a wide operating range. Variable geometry is achieved by pivoting the diffuser vanes to match the exit angle of the flow from the impeller and by adjusting the mechanical diffuser throat area. These adjustments permit greater flow under choke conditions while reducing the flow at which surge occurs. Choke flow is increased by causing the diffuser throat area to be larger at this condition. The flow rate at which surge occurs is reduced when the diffuser throat area is reduced by pivoting the diffuser vanes to match the more tangential exit flow angle from the impeller at the lower flow conditions.
There are two major drawbacks to a mechanically variable geometry system. First, a control system is required to move and fix the positions of the diffuser vanes under the various operating conditions. Second, it is difficult to seal the edges of the movable diffuser vanes which is necessary to avoid a loss in efficiency.